This invention relate generally to systems for time compression and expansion of signals. More particularly, the invention relates to a signal time compression and expansion system wherein, in restoring a frequency spectrum of a signal reproduced from a recording and reproducing apparatus to the original at a speed different from that in recording, the reproduced audio signal is divided into intervals made to coincide with a pitch period thereof and, after being frequency compressed or expanded, is connected together thereby to obtain a restored signal whose formant frequency is equal to that of the original signal.
It has been common practice in recording audio signals on a recording medium at ordinary speeds to reproduce the recorded signals at higher or lower speeds or to quicken or retard the tempo of reproduced speech voices as required. This method has sometimes been employed in consolidating the recorded speech contents delivered at a lecture meeting or at a round-table discussion, in the case of translating into a foreign language by listening to a recorded and reproduced discourse spoken in another language, in language lessons, and in other applications. This method has been used sometimes by busy persons and blind persons.
Merely switching one tape speed to another of an ordinary tape recorder may succeed in changing the tempo of a recorded speech, but this varies the pitch of voice sound and tone quality as well. This makes a reproduced speech indistinct and the speech content difficult to comprehend. Thus, there arises the need for a method whereby the tempo of a talk can be varied without accompaniment of variation in tone or sound quality.
There has been conventional method for meeting such a demand which comprises extracting a part of the original signals and slicing the remainder to such a degree that the speech content can still be understood when shortening the reproduction time interval, while inserting blank portions or reproducing the same portion repeatedly when the reproduction time is to be prolonged. With this conventional method, write-in or store and read-out operations are performed alternately and repeatedly by use of a single memory device. For this reason, the reproduced signal is liable to become intermittent or discontinuous, and its waveform envelope becomes at times a so-called tone-burst waveform.
A problem accompanying this method is that the tone quality of a reproduced sound is of poor quality accompanied by disturbances and interruptions.
Another problem encountered in the practice of this method is that memorized or stored and readout signals occupy only a part of the input signal, and in the case where a tape is played back at a speed which is twice the recording speed, for instance, stored and readout signals correspond to only one-third of the palyback signal, resulting in a great information loss, and hence the articulation of the reproduced voice is considerably sacrificed.